Answer:
 1.9 km​
Explanation:
The equation for vertical motion is ...
 h(t) = -4.9t^2 +26t +20
This will have a zero near t = 5.988 seconds.*
The horizontal distance traveled in that time is ...
 (310 m/s)(5.988 s) ≈ 1856 m ≈ 1.9 km
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* The root of a quadratic can be found numerous ways. We choose the quick and easy: let a graphing calculator show it to you.